I deal a lot with air compressors in my job and know that compressors are designed on paper for a specific inlet temperature, pressure, and relative humidity: often the AMSE standards of 68 deg F, 14.7 PSIA, and 36%RH.

I'd guess that automotive designers would rate their engine designs using more realistic under the hood figures. Does anyone know what the temperature and pressure (absolute) ratings are?

I ask because hot inlet air costs 2% in air mass input for each 10°F increase in air temperature. Likewise, dirty inlet filters lose .7% in air mass input for each .1 PSIA drop in inlet pressure.

I know a lot of people are using cold air intakes to improve engine breathing, but depending on the design temperature used by Subaru, dirty filters and long CAI tubes might cause more losses than the warmer air.

Just wondering,
Air

PS: If this should have been posted in a different forum, let me know and I'll move it.

That's why hardly anyone uses CAI on Subarus ;)

I understand that a lot of people are using CAI's, but there seems to be little if any agreement on their effectiveness.

If inlet air temperature were the only consideration then any CAI should work well as they are all similar in design. However, after reading many posts this doesn't seem to be the case. A lot of users report little or no improvement or in a few applications, a loss in performance.

Since almost all CAI's connect to the same points in the various cars, and since they all use similar size tubing and bends, why the variations unless it is the different filters used?

I have seen the article on Cobb's site that talks about just removing the snorkus and leaving the stock air box cover with good results - > "If you want the noise and slightly better spool-up, remove the silencer in the fenderwell and toss a panel filter in the stock air-box. On the stock turbo you will make the same power as you will with an aftermarket intake".

I'm considering modifying the air box cover by cutting out the box and leaving the edge to use as a frame to hold the panel air filter in the inlet plenum. This would essentially eliminate any inlet pressure losses on the inlet except for the filter itself. A small scoop could be used to provide make-up air to the vicinity of the inlet to help cool the incoming air, but given Cobb's note on using the stock air box, this is probably not needed.

BTW, my questions are aimed at NA applications.
Air